Silver salt of fatty acid treated with `I_2`/`C Cl_4`. If the ratio of silver salt & iodine is 1:1 then alkyl iodide is formed,if the ratio is 2:1 then ester is formed.Why?

Hunsdiecker reaction

The decarboxylation of the silver salt of a carboxylic acid with one molar equivalent of bromine or iodine to form an alkyl halide is called the Hunsdiecker reaction.

`"R-COOAg" + "I"_2 → "R-I" + "CO"_2 + "AgI"`

The mechanism of the Hunsdiecker reaction probably involves a radical chain reaction.

Step 1. Formation of the acyl hypoiodite

`"RCOO"^"-""Ag"^"+" + "I-I" → "RCOO-I" + "AgI"`

Step 2. Initiation

The acyl hypoiodite undergoes homolytic cleavage.

`"RCOO-I" → "RCOO·" + "·I"`

Step 3. Propagation

The acyl radical undergoes decarboxylation to form an alkyl radical.

`"RCOO·" → "R·" + "CO"_2`

Step 4. Propagation

The alkyl radical reacts with the acyl hypoiodite.

`"RCOO-I + ·R" → "RCOO·" + "R-I"`

Simonini reaction

The reaction of the silver salt of a carboxylic acid with two molar equivalents of iodine to form an ester is called the Simonini reaction.

`"2RCOOAg + I"_2 → "RCOOR" + "CO"_2 + "2AgI"`

In the mechanism, the alkyl halide that is formed in a Hunsdiecker type reaction probably undergoes an ionic substitution with the excess silver salt.

`"R-X + RCOO"^"-""Ag"^"+" → "RCOO-R" + "AgBr"`